The present invention relates to a method and system for measuring an article formed from two or more parts, in particular to a method of measuring a component comprising two or more parts arranged along a centre line or axis, such as a multi-stage bladed drum from a gas turbine.
Bladed discs, single components incorporating both a rotor disc and blades, have become popular in turbomachinery, and particularly in the aerospace industry. By incorporating the blades and the supporting disc or ring in a single component, weight savings that can be achieved over more conventional arrangements where individual blades are mechanically fixed to a disc at their roots.
Two or more bladed discs can be joined together to form multi-stage compressor drums for use within a turbine or engine, each bladed disc providing the blades for one stage
Damage can occur to turbine blades during operation, and operational temperatures and loads are so high that blades can also become stretched without any external influence. To ensure reliable operation, regular servicing of turbines is important. One drawback of bladed discs is that servicing becomes more complicated. More conventional designs allow for the replacement of one or more individual blades within the assembly as required, but the unitary nature of bladed discs removes this option. Instead, to avoid disposing of an entire bladed disc or even an entire bladed drum when a single blade is damaged, adaptive machining is used to repair or grind down and replace damaged or faulty blades. This requires inspection of the component and comparison with an ideal model.
Inspection, adaptive machining and repair of components can be achieved by a number of techniques. Since the advent of Computer Aided Design (CAD) one technique has been to compare measured details of an actual article with a nominal CAD model of the same article and use software to determine differences between the two and, in some cases, provide instructions to automated tools for the adaptive machining or repair process.
In order to achieve comparisons of this type it is necessary to generate an accurate three dimensional model of the article in question and then precisely align the generated model with a corresponding nominal CAD model. Scanning technologies such as photogrammetry and structured light scanning allow an accurate representation of an article to be obtained, and unique identifiable reference markers, located on or near the article during the scanning process, additionally allow for the recording of data relating to the position and orientation of the article. This data can then be used to ensure precise alignment of the features of the measured article with equivalent features of the nominal CAD model so that an accurate comparison can be made.
For a single bladed disc this alignment and comparison is relatively straightforward. The individual blades are consecutively numbered around the disc or ring for reference and ease of identification of individual blades, with blade one being treated as ‘Top Dead Centre’ (TDC). All that is required is the correct identification of blade one at the beginning of the scanning procedure to allow the alignment of TDC of the measured component with TDC of the model.
However, the situation is far less straightforward where a number of bladed discs are joined together to form a bladed drum.
Varying requirements of different turbines mean that numerous different combinations of stages are possible, so it is common to model the stages individually rather than modelling complete drum assemblies. This can lead to problems during inspection because there will not be a single TDC for the drum unless blade one for all stages is aligned during manufacture. When the blades are not aligned repeatably and the timing relationships between stages are unknown, it makes inspection and repair or adaptive machining very difficult.
The solution has been to scan each stage individually and compare with its own equivalent model for inspection and machining. However, this approach requires that the drum is repeatedly moved between scanning and machining, requiring multiple set-ups which are time consuming and increase the risk of further damage to the blades.
It would be beneficial, therefore, if the scanning process could be performed for all stages of the assembly before any comparison or adaptive machining were performed.
One solution would be to ensure that all stages on every bladed drum are timed (arranged with blade one of each stage aligned) during assembly. However, certain manufacturing techniques, such as inertial friction welding, make it near impossible to align features, such as turbine blades, between stages of a multi-stage assembly. Inertial friction welding involves rotating two components relative to one another about a common axis to generate heat on their end faces, melting the end faces and then quickly stopping the relative rotation and applying pressure to weld the two drums together. It will be appreciated that any control of rotational alignment of components is extremely difficult to achieve.
Even where alternative approaches, such as Electron Beam Welding, would allow timing of stages, this can still be time consuming and difficult to achieve. Furthermore, in some case an alternative alignment may be preferable for optimum balance and performance of the turbine.
A conflict therefore exists between what is desirable for inspection and repair of components and what is desirable or achievable in their manufacture.
It is an aim of the present invention to overcome or mitigate this conflict.